Field of the Invention
The present invention relates to an encoder and a method of adjusting magnetic fields of the same, and more particularly, it relates to a magnetic encoder which can obtain detection output of high resolution with relatively simple structure.
In an automatic control system, various position sensors and rotation angle sensors are employed in order to obtain position feedback signals and the like. Such sensors are prepared as a rotary encoder, a linear encoder, a synchronous sensor, a resolver and the like. Within these sensors, the rotary encoder is widely used for detecting arm positions in an industrial robot and pivot positions in an NC machine tool.
As is well known in the art, there have been provided optical type and magnetic type as rotary encoders, and the magnetic rotary encoders classified into gear type and drum type ones. In these encoders the optical rotary encoder can be improved in resolution by reducing slits provided on a rotating panel in width and array pitch. However, since the slits are limited in manufacturing accuracy, the improvement in resolution is restructed. Further, misalignment etc. may be caused between the rotating panel and a fixed panel by temperature increase, whereby the improvement in resolution is restricted.
On the other hand, a gear type magnetic rotary encoder can be manufactured at a low cost. However, when the gear is low in manufacturing accuracy of tooth profiles, the teeth are not uniform in configuration and hence tooth pitches cannot be improved in accuracy. Thus, the resolution is restricted also in the gear type encoder.
Description is now made of a drum type encoder. FIG. 1 is a conceptual diagram showing a conventional drum type encoder. Referring to FIG. 1, the drum type encoder comprises a drum 1 fixed to a rotary shaft (not shown) and a magnetic sensor such as a magneto-resistance dropping device 2 provided so as to face the surface of the drum 1 across an appropriate gap g. The outer peripheral portion of the drum 1 is so magnetized that magnetic poles N and S are alternately arrayed along the circumferential direction. Upon rotation of the drum 1, therefore, the magneto-resistance dropping device 2 outputs a pulse train in a cycle corresponding to the array pitch of the magnetic poles. Thus, the pulses are counted to obtain detecting output expressing the angle of rotation of the drum 1.
In such a drum type encoder, resolution is improved by reducing the array pitch of the magnetic poles N and S. However, following the reduction in the array pitch of the magnetic poles, an effective distrubution range d of magnetic fluxes .phi. is decreased in the radial direction of the drum 1 as shown at FIG. 2 (a) and (b). Therefore, the gap g between the magneto-resistance dropping device 2 and the magnetized surface of the drum 1 must be reduced in order to improve resolution. However, when the gap g is excessively reduced, relative errors (.DELTA.g/g) are considerably increased even if the gap g is in a small error .DELTA.g. Thus, small variations in the gap g lead to relatively large variations in detection output of the encoder, to lower detection accuracy.
Thus, the conventional encoders are restricted in formation of slits and tooth profiles and subdivision of magnetization, and hence proposed is improvement not in mechanical structure but in signal processing. Such improvement is disclosed in, e.g., Japanese Patent Laying-Open Gazettes Nos. 58-85113(1983) and 59-211822(1984), in which data obtained from encoders are further subdivided by an interpolation method to provide detecting output of high resolution.
However, extremely high resolution cannot be attained only by the said improvement in signal processing, but improvement in mechanical structure is required. Description is now made of such a case.
In most of conventional industrial robots and various machine tools, driving force from a motor is supplied to arms and pivots through a decelerator. A rotary encoder is coupled with the motor to obtain positional data on the arms and pivots by detecting rotation angles of the motor. When errors are caused in detection of the rotation angle of the motor, the values of the errors are multiplied by the moderating ratio of the decelerator to provide errors in positions of the arms and pivots. Needless to say, the moderating ratio is smaller than one. Therefore, the positional errors in the arms and pivots are smaller than the errors in the encoder, and hence the detection errors of the encoder substantially exert no bad influence on a control system.
However, increasingly employed in recent years are a decelerator having a moderating ratio approximate to one and a motor requiring no decelerator such as a direct-drive motor. In such case, detection errors in an encoder directly become positional errors in arms and pivots. Thus, awaited is provision of an encoder which can mechanically attain high resolution.
For example, resolution higher than 1,000,000 pulse/rev. is required in an industrial robot employing a direct-drive motor. However, an encoder of such high resolution is expensive and the absolute accuracy thereof is not sufficient for practical use due to the various dificulties as hereinabove described. Therefore, a high-priced synchronous resolver is generally employed in such an industrial robot.
Description is now made on magnetic field adjustment in the magnetic encoder. In the magnetic encoder, magnetic fields generated in, e.g., a rotor side of a motor are detected by magnetic sensors provided in a stator side. The magnetic sensors are in intrinsic detection characteristics responsive to the types thereof, in which detection characteristics are linear only within a range of specific magnetic field strength. Rotation angles can be correctly detected by detecting the magnetic fields under such condition that strength of the magnetic fields in the positions in which the magnetic sensors are arrayed is in regions with such high linearity. Thus, magnetic field adjustment is required in the magnetic encoder.
When the magnetic fields are generated by magnetization as shown in FIG. 1, such magnetic field adjustment can be performed by correcting the magnetization value. When magnetic substances are magnetized through currents to detect magnetic fields thereby generated, magnetic fields can also be adjusted by correcting the current values. Magnetic field adjustment can furthr be performed by correcting assembly sizes of such magnetic field generating machanisms and magnetic sensors.
However, particularly when magnetic fields are generated by magnetization, correction of magnetization is considerably difficult in practice since such magnetization is corrected by repeating magnetization and demagnetization. Further, assembly sizes of respective components cannot be easily corrected. Thus, such methods are not sufficiently practical.